This invention relates generally to cathode ray picture tubes. It is specifically addressed to an electron gun for use in small, short picture tubes intended primarily for dashboard installation in vehicles.
An electron gun for use in a cathode ray tube intended for a vehicle (a "VCRT"), must meet severe operational and physical constraints. These constraints are best explained by comparison of the VCRT with the standard cathode ray tube of five-inch diagonal measure intended for use in small television receivers.
Peak brightness of the VCRT, for example, is preferably of the order of ten times the brightness (under typical operating conditions) of a cathode ray tube used in a consumer product television receiver. This magnitude of brightness is required to provide adequate image legibility in the high ambient light environment encountered during day-time operation. This includes the possibility of direct sunlight falling on the face of the tube. High brightness also compensates for brightness loss resulting from the use of a neutral density filter needed to increase image contrast. Factors which contribute to high brightness are primarily high current density of the electron beam, and high phosphor efficiency.
Space constraints, especially in behind-the-dashboard installations, place particular restrictions on the overall depth of the VCRT. The total length of the tube is restricted while the deflection angle is increased to more than 100 degrees to achieve a three by eight aspect ratio. The typical five-inch diagonal television tube has a 70 degree deflection angle. In the VCRT this increase in deflection angle maintains overall tube depth, but adversely affects corner spot size performance. The gun length is also restricted by the total tube length constraint. The problem which the gun designer faces, then, is to maintain a standard of center and corner spot size performance in spite of a large deflection angle and gun length constraints.
Quick comprehension of a displayed message for decision making when a potentially hazardous situation is confronted is an important requirement in VCRT's, so the resolution of the tube must be high. The degree of resolution is a function of beam spot size in cathode ray tubes. The center-screen spot size of a VCRT is preferably less than 60 mils at a beam current of 1.5 mA, with the anode potential at 12 kV. By comparison, the spot size for satisfactory performance in a five-inch diagonal CRT is about 80 mils at operating conditions similar to those for a VCRT.
Electron guns used in television picture tubes generally consist of two basic parts: (1) an electron beam source, and (2) a lens for focusing the electron beam on the phosphor screen of the cathode ray tube. Most commerical focus lenses are electrostatic and consist of discrete, conductive, tubular elements which are arranged in sequence on an axis. These electrodes are supplied with predetermined voltages which establish the electrostatic focusing field. A main objective in designing an electron gun is to produce a small, symmetrical beam spot on the tube's cathodoluminescent screen.
The electron beam focus lens of the novel electron gun is classified as a unipotential according to the teachings of present invention. That is, it consists of three electrodes with the center electrode being the focus electrode and the first and third electrodes being at the same potential. Potentials as applied in color television cathode ray tubes are typically 25 to 30 kV on first and third electrodes and zero volts on the center electrode. The voltage on the center electrode is often variable. The prefix "uni" denotes that the potential of the final electrode is the same as that on the initial electrode.
A second type of electron beam focus lens is known as a bipotential. It consists of two electrodes. The first is the focus electrode and typically operates at 6 to 8.5 kV for color television cathode ray tubes. The second electrode is an accelerating electrode and normally operates at potentials of 25 to 30 kV for color television.
In U.S. Pat. No. 4,287,450, Kawakami et al. discloses an electron gun assembly which has " . . . a cathode, an apertured modulator electrode located close to the cathode for controlling the intensity of the electron stream, a first anode in the form of an apertured metal member located close to the modulator, an apertured second anode located close to the first anode and a third cylindrical focusing anode adjacent to the second anode . . . ". The modulator electrode and first anode are supplied with positive potentials which form a cross-over between them. The second anode is supplied with a lower positive potential so that the electron stream emitted from the cathode is formed into a narrow beam, and enters the third anode at a small angle of beam spread. The beam is said to come to focus on the screen of the tube to produce a spot of small cross-sectional area. The effect is to cause the beam to diverge under the influence of the second anode as a result of the relatively lower potential, and converge in the field of the third focusing anode, which is at a relatively higher potential.
Hawken et al in U.S. Pat. No. 4,277,722 discloses a cathode ray tube having two electron lenses in combination which cause a second cross-over of the electron beam between the two lenses. One of the two electron lenses has a variable voltage which dynamically controls the location of the beam cross-over in order to focus the beam onto a display screen at any location away from the screen center. The second cross-over is located within the final electrode of the electron gun. A focusing amplifier provides a potential in the range of 100 to 500 volts for adjusting the focus of the beam.
The unipotential type electron gun has achieved less commercial success than the bipotential-type gun primarily because of its prediliction to arcing in the lower end. The difference in potentials of the final prefocusing electrode and the initial accelerating electrode of a unipotential type gun may be as much as 30,000 volts in guns used in some color cathode ray picture tubes. This great difference in potentials, coupled with the relatively close spacing of the two electrodes, is an incitement to destructive arcing.
Representative examples of electron guns of the unipotential type include those disclosed in Hasker et al.--U.S. Pat. No. 3,919,583; Bortfeld et al.--U.S. Pat. No. 4,124,810; and Sakurai et al.--U.S. Pat. No. 4,232,246.